The spatial heterodyne spectrometer (SHS) design (FIG. 1 is an exemplary illustration) is compatible with pulsed laser excitation and offers several advantages including high spectral resolution, limited by the diffraction gratings, in a very small form factor; very high optical etendue and thus high throughput; and demonstrated high resolution in the UV. Applications of spatial heterodyne spectrometers (SHS) outside of astronomy are still relatively few; however a UV absorption SHS spectrometer has been successfully demonstrated in space on STS 112.6 The basic SHS design is like a Michelson interferometer but with tilted diffraction gratings and no moving parts and like a Michelson there is no input slit. The SHS offers a wide acceptance angle at the grating (e.g., 1° or 10° using field widening prisms) and thus a wide-area measurement capability, high light throughput and imaging capabilities.
The spatial heterodyne spectrometer was only recently described for Raman applications, likely because SHS technology has been focused on astronomical remote sensing and because most systems are designed for a very small spectral band pass. However, the SHS spectrometer design is ideal for Raman, especially deep-UV Raman spectroscopy. The use of diffraction gratings in the SHS interferometer design provides much higher resolution in the UV, even with the relatively few samples provided by the charge-coupled device (CCD) detector, and good control over the spectral range.
As described by the inventors in U.S. application Ser. No. 13/654,924, incorporated by reference herein, the Raman scattered light is collected and collimated, then filtered by the two holographic filters to remove laser scatter from the Raman signal (an exemplary embodiment is shown in FIG. 2). The filtered, collimated light passes through a 25-mm aperture into the SHS. Light entering the SHS is split into two beams by the 50/50 beam splitter. The separated beams strike the tilted diffraction gratings, are diffracted back along the same direction, re-enter the beam splitter, and recombine. The grating tilt angle defines the Littrow wavenumber, σL, the wavenumber at which both beams exactly retro-reflect, producing no constructive or destructive interference and therefore no fringe pattern at the detector. For any wavelength other than Littrow, the recombined light produces a crossed wave front, of which the crossing angle is wavenumber dependent, and produces an interference pattern at the interferometer output, which is the Fourier transform of the Raman spectrum. The interference pattern imaged onto the ICCD or CCD detector produces an image of vertical fringes. The number of fringes, f, across the ICCD is related to the Littrow wavenumber by Eqn. 1:f=4*(σ−σL)*tan θL,  Eqn. 1
where f is in fringes/cm, σ is the wavenumber of interest, σL is the Littrow wavenumber and θL is the Littrow angle. Bands with larger wavenumber shifts produce more closely spaced fringes. Because of the symmetry in this equation, spectral features at wavenumbers both higher and lower than Littrow overlap on the detector. In the case of Raman spectra, this can cause overlap of Stokes and anti-Stokes bands if the Littrow wavelength is set near the laser excitation wavelength. However, this overlap can be avoided by tilting one grating, producing a rotation of the fringe pattern clockwise for bands at wavenumbers below the Littrow wavelength and counter-clockwise for bands above Littrow. A 2D Fourier transform (FT) of the resulting interferogram recovers both spectra, above and below the Littrow wavelength. The use of a 2D FT to recover independent spectral information in the vertical and horizontal dimensions of the interferogram can be used to double the spectral range of the SHRS without additional samples and it can also be used for 2D imaging as described below.
A small UV Raman spectrometer is a particular technical challenge using dispersive (grating) approaches and requires large spectrographs and very narrow slits to achieve the spectral resolution required for many applications. The heterodyne approach of the SHS has only a weak coupling of resolution and throughput, so a high resolution UV SHRS can both be small, and employ a wide aperture to maximize throughput. The SHRS measures all optical path differences in its interferogram simultaneously with a detector array, so the technique is compatible with gated detection using pulsed lasers, important to reject ambient background and mitigate fluorescence (already low in the UV) that is encountered for most “real” samples. In the SHRS, the spectrum is heterodyned around the laser wavelength, making it particularly suitable for Raman measurements.
As such, it would be desirable to provide suitable systems and methods for small sized Raman spectroscopy to measure biomarkers and other samples of interest such as minerals, water, CO2 ice, or the like.